perf tools: Fix use-after-free in help_unknown_cmd()

Currently perf aborts when it finds an invalid command.  I guess it
depends on the environment as I have some custom commands in the path.

  $ perf bad-command
  perf: 'bad-command' is not a perf-command. See 'perf --help'.
  Aborted (core dumped)

It's because the exclude_cmds() in libsubcmd has a use-after-free when
it removes some entries.  After copying one to another entry, it keeps
the pointer in the both position.  And the next copy operation will free
the later one but it's the same entry in the previous one.

For example, let's say cmds = { A, B, C, D, E } and excludes = { B, E }.

  ci  cj  ei   cmds-name  excludes
  -----------+--------------------
   0   0   0 |     A         B       :    cmp < 0, ci == cj
   1   1   0 |     B         B       :    cmp == 0
   2   1   1 |     C         E       :    cmp < 0, ci != cj

At this point, it frees cmds->names[1] and cmds->names[1] is assigned to
cmds->names[2].

   3   2   1 |     D         E       :    cmp < 0, ci != cj

Now it frees cmds->names[2] but it's the same as cmds->names[1].  So
accessing cmds->names[1] will be invalid.

This makes the subcmd tests succeed.

  $ perf test subcmd
   69: libsubcmd help tests                                            :
   69.1: Load subcmd names                                             : Ok
   69.2: Uniquify subcmd names                                         : Ok
   69.3: Exclude duplicate subcmd names                                : Ok

Fixes: 4b96679170c6 ("libsubcmd: Avoid SEGV/use-after-free when commands aren't excluded")
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250701201027.1171561-3-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

diff --git a/tools/lib/subcmd/help.c b/tools/lib/subcmd/help.c
index 8561b0f01a2476908bd2bfd73dfd8677be959ef2..9ef569492560efd7c27a6223d4050ea92b1d533c 100644 (file)
--- a/tools/lib/subcmd/help.c
+++ b/tools/lib/subcmd/help.c
@@ -9,6 +9,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 #include <dirent.h>
+#include <assert.h>
 #include "subcmd-util.h"
 #include "help.h"
 #include "exec-cmd.h"
@@ -82,10 +83,11 @@ void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes)
                                ci++;
                                cj++;
                        } else {
-                               zfree(&cmds->names[cj]);
-                               cmds->names[cj++] = cmds->names[ci++];
+                               cmds->names[cj++] = cmds->names[ci];
+                               cmds->names[ci++] = NULL;
                        }
                } else if (cmp == 0) {
+                       zfree(&cmds->names[ci]);
                        ci++;
                        ei++;
                } else if (cmp > 0) {
@@ -94,12 +96,12 @@ void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes)
        }
        if (ci != cj) {
                while (ci < cmds->cnt) {
-                       zfree(&cmds->names[cj]);
-                       cmds->names[cj++] = cmds->names[ci++];
+                       cmds->names[cj++] = cmds->names[ci];
+                       cmds->names[ci++] = NULL;
                }
        }
        for (ci = cj; ci < cmds->cnt; ci++)
-               zfree(&cmds->names[ci]);
+               assert(cmds->names[ci] == NULL);
        cmds->cnt = cj;
 }